This invention relates to a method for detection of switching device failure conditions in a multilevel converter.
Multilevel converters (e.g. neutral point clamped converters) are generally used in high power industrial applications such as variable speed drive (VSD) systems or in energy conversion applications such as a solar (or photovoltaic) power generation systems, wind turbine generators or marine and hydrokinetic power generation systems. The general function of the multilevel converter is to synthesize a sinusoidal voltage by several levels of voltages, typically obtained from capacitor voltage sources. A three level converter includes two capacitor voltages in series with the center tap as the neutral. Each phase leg of the three-level converter has two pairs of switching devices in series. The switching device is bidirectional in current, often realized as anti-parallel connection of a unidirectional electronic switching device (e.g. IGBT, IGCT) and a diode (free-wheeling diode).
The switching devices in a multilevel converter receive high electrical and thermal stress during short-circuit conditions or during turn-off switching of a clamped inductive load. For example if there is large power loss within the switching device due to electrical stress or overcurrent, the switching device overheats and it may lead to thermal breakdown of the switching device. Similarly, when an inductive load or inductive current is switched off, the switching device may observe overvoltage resulting into voltage breakdown of the switching device. Once a switching device is broken down or destroyed due to overheat or overvoltage, without protective measures, a chain reaction may occur resulting into destruction of the entire multilevel converter bridge. Thus, it is important to detect failures in switching devices in multilevel converters. One example of a measurement used for detecting switching device failure in IGBT converters is a desaturation circuit. However, methods using the desaturation circuit detect a failure condition only when the corresponding switching device is turned ON and do not provide any signal when the gate drive signal to the switching device is turned OFF. Thus, the desaturation circuit does not detect all possible short-circuit failure conditions.
Therefore, it is desirable to provide a method and a system that will address the foregoing issues.